In many applications including the formation of intended thin-film on semiconductor substrates in a chemical vapor deposition (CVD) furnace, it is often desirable to control the quantity (i.e. mass) of gas flow of a preselected gas per unit time to a gas utilization device, such as the CVD furnance. A heretofore known mass-flow controller employed for such a purpose includes a monolithic cylindrical steel housing having a central bore defining a longitudinally extending fluid-flow conduit. A flow restrictor is transversely positioned in the central bore and intermediate the ends of the cylindrical housing for both providing a pressure head and for limiting the fluid-flow to within prescribed bounds therethrough. A pressure sensing tube responsive to the pressure head is externally mounted to the housing with one of its ends opening to the central bore and upstream of the flow restrictor, and with the other of its ends opening to the central bore and downstream of the flow restrictor. A heating coil is centrally provided around the sensing tube for heating the gas that flows therethrough, and upstream and downstream temperature sensing coils (RTD's) are provided around the sensing tube to respective sides of the heating coil.
The mass of the gas that flows in the sensing tube absorbs the heat produced by the heating coil, and in such a way that the quantity of heat absorbed therefrom uniquely depends upon the specific heat of the individual gas to be mass-flow controlled. The difference in temperature thus sensed by the upstream and the downstream temperature sensing coils is representative of mass-flow rate in the sensing tube, which is proportional by a known proportionality constant to the mass-flow rate in the longitudinally extending fluid-flow conduit of the cylindrical housing of the mass-flow controller.
The heretofore known mass-flow controllers however, among other disadvantages, must be individually calibrated for each gas to be mass-flow controlled, and for a particular range of flow rate for each such gas, necessitating an undesirably large inventory of, and associated cost for, individual mass-flow controllers respectively dedicated to a particular gas and range of flow-rate. Furthermore, contaminants present in the controlled gas and on the walls of both the sensing tube and the housing of the mass-flow controller commonly build-up and occlude not only the sensing tube but also the flow restrictor shifting the proportionality constant and therewith producing undersirable mass-flow control error. Moreover, the replacement and repair of an occluded sensing tube and/or flow restrictor necessitate the disconnection of the heretofore known mass-flow controller, resulting in considerable down-time with consequent labor cost, replacement cost, and lost system throughput revenue.